This is a non-treatment study; however, volunteers receive hepatitis A and tetanus vaccinations. Numbers of phenotypically naive CD4+ cells (CD45RA+/CD62L+) are compared between healthy, HIV-seronegative volunteers and HIV-seropositive patients of A5015. An array of assays to assess baseline differences in immune function between these study populations are performed. Expression of markers of activation are compared by measuring the coexpression of HLA-DR+/CD38+ and CD28+ on CD4+ and CD8+ lymphocytes between these populations. To investigate possible age-associated differences in apoptosis, Fas (CD95+) expression is measured on CD4+ and CD8+ T cells by flow cytometry, and spontaneous apoptosis is assessed using the propidium iodide method. DTH hypersensitivity to skin test antigens, lymphocyte proliferation to mitogens, soluble antigens, recall antigens, and neoantigens are compared between the 2 populations. Antibody responses to vaccination with tetanus and hepatitis A are assessed. Finally, thymic size as measured by CT scan and the frequency of T cells that contain TRECs is compared between these 2 populations.